In the case of a coating, which is able to break down water into hydrogen and oxygen, there is a risk that the coating reacts with the water content present in the ambient atmosphere, in particular in the form of water vapor, by forming atomic hydrogen. There is a risk thereby that this hydrogen and/or hydrogen, which is already present in the ambient atmosphere, enters the material of the base body and leads to a charging of the base body with atomic hydrogen in an undesirable way. For a hardened base body, which is charged with hydrogen, there is a risk of a hydrogen embrittlement, whereby the maximum sustainable tension is reduced significantly. This can also lead to a hydrogen-induced brittle fracture of the body produced from the base body and hardened by hot forming, in particular in response to tensioning for the purpose of installation or joining, for example by means of welding.
There is a risk of the input of atomic hydrogen into the material of the base body in particular during austenitization of the base body, because the heating of the precoated base body favors a reaction of the coating with the water, which is present in the ambient atmosphere, by forming atomic hydrogen.
Under this aspect, all metallic coatings, which are able to reduce water vapor by forming hydrogen in response to elevated temperatures, as they appear during austenitizing, for example, are to be considered as being problematic with regard to a hydrogen charging of the base body.
The problem of the formation of atomic hydrogen by a reaction of the coating with water vapor present in the atmosphere appears in particular in the case of aluminum coatings or aluminum-containing coatings, such as zinc aluminum, aluminum silicon or zinc magnesium or also combinations of zinc, aluminum and/or magnesium, which break down water vapor into hydrogen and oxygen in response to heating.
A further problem occurs in particular in the case of aluminum-containing coatings, for example aluminum silicon-coated sheets, in the case of which the coating is in contact with other materials in response to an elevated temperature. This is so, for example, when austenitizing and the associated heating of the material takes place in a continuous furnace and when the coating comes into contact with the rollers of the furnace, which are preferably made of a ceramic material. The rollers can be transport rollers or also rollers for a press hardening, for example. Due to the small thickness of the oxidized layer of the coating, the oxidized layer of the coating might be penetrated in response to mechanical stress to the above-described base body. The coating might furthermore also melt partially. The rollers thus contact the melt of the coating, which can lead to an infiltration of the rollers with the melt of the coating, among others. This contact can lead to damages to the transport rollers and finally to a breakage of the rollers, in particular in the case of an aluminum silicon coating.
In response to a breakage of the oxide layer in a furnace, the coating furthermore comes into contact with the furnace atmosphere, which is present in the furnace, which, in turn, leads to the formation of hydrogen by reaction of water vapor present in the furnace atmosphere with the melt of the coating, whereby the produced body hardened by hot forming, has an inadmissibly high content of diffusible, atomic hydrogen. This must be considered as being critical in particular in the case of furnaces, in the case of which significant quantities of water vapor are present in the furnace atmosphere.
A method for producing a coated body hardened by hot forming made of a base body made of metal, which is precoated with a metallic material, is known from EP 2 507 503 A2, whereby the precoated base body is austenitized in a method step. To ensure a sufficient oxidation of the coating while simultaneously reducing the risk of a hydrogen embrittlement, it is proposed to heat a printed circuit board, which is provided with a coating, in a furnace. A metallic alloy layer is formed on the printed circuit board at least area by area. The atmosphere inside the furnace is controlled by the supply of pretreated air, in that the pretreated air is dried prior to being supplied. The portion of dissolved water in the form of water vapor is thus reduced inside the furnace atmosphere, an less water, which can be broken down, is present in the atmosphere of the furnace. A possible hydrogen embrittlement of the printed circuit board hardened by hot forming is thus reduced by means of hydrogen, which enters the material.